Ion trap magnet



Dec. 25, 1951 1. E. LEMPERT 2,580,355

ION TRAP MAGNET Filed 001',- 8, 1949 Fig.

HORIZONTAL AND VERTICAL DEFLECTION CIRCUITS SIGNAL AMPLIFIER w VIDEO 46 \QD 43 44 PICTURE INVENTOR.

' BRGHTNESS Y mw/va E. LEMPERT CONTROL v Patented Dec. 25, 195?.

UNH'E'E ION 'rnar MAGNET Irving E. Lempert, Maywood, N. J., assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. 3., a corporation of Delaware Application October 8, 1949, Serial No. 120,308

Claims.

My invention relates to magnetic structures such as are used to separate electrons from negative ions in a cathode ray television picture tube.

Negative ions, often present to an appreciable extent in cathode ray tubes, cause damage to fluorescent screens, unless diverted or trapped. In general, ion traps utilize the principle established by Dempster and the Astons that negatively charged particles are impelled in the same path by electrostatic forces regardless of the mass of the particles, and that a magnetic field in conjunction with the electrostatic field provides a means for separating particles from one another in accordance with their relative masses. By deflecting or bending the ion and electron beams or paths, the ions may be trapped while the electrons continue to the fluorescent screen.

Television receivers employing the trap obtain operating potentials for the cathode ray tube from the main power supply which often varies considerably in magnitude at various times of day or in short surges caused by the switching on or off of heavy electrical loads. Resulting variations in the amplitude of the alternating voltage cause similar variations in the electrode potentials. Although the strength of the magnetic field to accomplish this separation of electrons from ions under normal and fixed conditions, is well understood by those skilled in the art, Variations in voltage cause an undesirable variation of the electrode potentials and the ion trap.

It is an object of this invention to provide an ion trap magnet which minimizes or entirely eliminates variations in picture quality due tothese changes of electrode voltages.

Other objects and features will be apparent from the following description, claims and accompanying drawings in which:

Figure l is a front elevational View, partl diagrammatical, of a magnetic structure embodying my invention.

Figure 2 is a side elevational View of a cathode ray tube having an ion trap using the magnet of Figure 1, partly broken away, and the accompanying electrical circuit shown schematically.

Referring to Figure 1, a yoke I2 of soft magnetic material is shaped to form pole pieces I3 which can be clamped on either side of the cylindrical neck of a cathode ray tube by means of a clamp screw I 4 of non-magnetic material which engages a tapped hole in the end l5 of the yoke I2. A magnetic field shown by dotted lines 56 is formed between the pole pieces, in part by an electromagnetic winding ll of copper wire surrounding a portion of the yoke, and in part by a permanent magnet I8 located across the yoke between the ends of the pole pieces I3. In accordance with this invention the magnetic field 16 should be provided by the electromagnet and by the permanent magnet in approximately equal amounts.

Referring now to Figure 2, and particularly the mechanical parts shown therein, a thermally activated cathode 22 emits electrons in a stream 23 the intensity of which is controlled by anegatively biased electrode or control grid 24. The electrons in the stream are then accelerated by a positive electrode 25. In the absence of a magnetic field these electrons would continue in a straight path impinging upon apart 26 of the first anode. A magnetic field is, however, provided by the magnetic structure of Figure 1 here shown with one pole piece broken away, the field being represented by the flux lines I6 of Figure 1. Because of the presence of the magnetic structure, the electrons are bent within the region of the first anode 25 and pass out through a hole 21 in the end of the first anode 25, in a beam 28 of electrons which then is focussed, deflected, and accelerated toward the fluorescent screen 29 providing a television picture area.

The negatively charged ions which form at the cathode 22 are also accelerated in a beam 23 by the first anode 25. These relatively heavy particles, however, are substantially unafiected b the magnetic field I6 and continue in almost a straight line 32 into the trap 26.

If the magnetic field I6 is too strong, the electrons willbe diverted too'far as shown by the line 33 and fall against a point 34 of the first anode structure. On the other hand, if the field I6 is weak the electrons will be diverted an insufficient amount and will fall in a beam 35 against the point 36 in the first anode.

In the electrical circuit of Figure 2 the cathode 22, the control grid 24, and the first anode 25 are connected to a source of direct potential consisting of a rectifier 42, filter condensers 43 and 44, a filter resistor 45, and a picture brightness control potentiometer 46, all deriving energy from a source of alternating voltage 41 which is normally the cycle public power supply. The power supply is subject to the variations previously mentioned. The electromagnetic winding I7 is shown connected in series with the potentiometer 46 so that any change in the voltages applied to the cathode ray tube electrodes will be accompanied by a proportional change in current in the electromagnet.

The manner of operation is as follows: If, as

3 an example, the line voltage changes, increasing for instance ten per cent, the potentials in the tube will increase, and the velocity of an electron travelling in the beam 23, will increase approximately five per cent, the velocity changing directly with the square root of the energy. If the strength of the magnetic field '16 did not change, the electron would be inadequately diverted and'would follow a path 35. It therefore is necessary to increase the strength of the magnetic field I6, to compensate for the increase in velocity, the amount of increase needed being five per cent.

This increase is provided by the electromagnet which draws approximately "ten per cent more current than before. were not present, and if no saturation of the iron within the yoke occurred, the strength of the field would increase by ten per cent. However, since approximately half of the energy in the magnetic field I6 is provided by .the permanent magnet, a ten .per cent increase in current in the electromagnet will result in'only a f ve per centincrease in the field in the gap, which is the required amount.

Although a condition has been described in which each magnet-contributes equally to :the field to produce the proper compensation for line voltage variationsrit has been found that certain structures require a small departure fromthis value to obtain an exact balance, this departure probably being caused-by partial saturation of the iron yoke. 'Ifhe yoke should be operated with the :fiux density of the iron well below saturation so that compensation over a wide range of line voltage variations is obtained.

Although a specific embodiment has been shown and described, thescope of my invention can best be understood fromthe accompanying claims.v

What is claimed is:

1. Apparatus of the type described comprising a cathode ray tubehaving-a cathode for emitting electrons, a plurality of electrodes for controlling said electrons, and an ion trap, a source of direct voltage for saidelectrodes, an electromagnet connected to said source; and actuated thereby, said electromagnet .being positioned to provide mag.- netic flux in the space between said electrodes to deflect said electrons away from said trap, and

a permanent magnet likewise positioned to provide in said space magnetic flux substantially equal to said flux provided-by said ,electromagnet, each said .magnetic flux being transverse to the path of .said electrons,

2. The device Eof claim 1 in which said electronragnet consists of copper winding surrounding a soft magnetic structure terminating in pole If the permanent magnet 4 pieces on either side of the neck of said cathode ray tube. 3. The device of claim 2 in which a soft magnetic structure links said pole pieces with both said electromagnet and said permanent magnet. 4. An ion trapmagnet for a television receiver having a cathode ray tube with an electron beam therein and a source of "direct potentials connected 'to'said tube, said'p'otential being obtained by rectifying alternating current from a supply subject to variations, comprising an electromagnet actuated by said rectified current and positioned to provide flux laterally across a gap through which said :beam passes, and a permanent magnet positioned to provide laterally across said gap --an amount of flux substantially equal to said fiux provided by said electromagnet.

5. In a television receiver, a cathode ray tube having an anode and an electron beam therein and a source of direct potential connected to said anode of said tube, which source is subject to variations, a magnetic structure for separating electrons from negative ions comprising'a yoke of soft magnetic material having pole pieces positioned laterally adjacent the neck of said tube on opposite sides, thereof, a permanent-magnet an electromagnet, both of said magnets being in magnetic circuit with said yoke and said pole pieces to induce a magnetic field in theneck of said tube normal to said beam, the components of field produced by each magnet being approximately equal and means for actuating said electromagnet in accordance with said variations of said source of direct potential.

IRVING E. LEMPERT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,181,850 Nicoll Nov. 28, 1939 2,200,039 Nicoll May 7, 1940 2,211,613 Bowie 1 Aug. 13, 1940 2,274,586 Branson Feb. 24, 1942 2,431,077 Pooh Nov. 18, 1947 2,433,682 Bradley 11-1 Dec. 30, 1947 2,442,975 Grundman June 8, 1948 2,456,474 Wainwright Dec. 14, 1948 2,460,609 Torsch Feb. 1, 1949 2,472,766 Woodbridge June '7, 1949 2,483,133 Gethmann Sept. 27, 19.49 2,499,065 Heppner 1- Feb. 28, 1950 2,500,455 Fisher ..1 Mar. .14, 1950 2,515,305 Kelar July 18, .1950 2,539,156 Ostreicher Jan. 23, 1951 

